Producing sustainable aviation fuel

Sustainable fuels

The table below shows the seven sustainable aviation fuel (SAF) pathways approved by ASTM International – an organisation that brings together hundreds of researchers, technical experts and scientists to determine the technical requirements to ensure fuel safety. Each pathway represents different processes for production depending on the type of feedstocks. 

The pathways all have potential benefits such as feedstock availability and cost, total carbon reduction, or processing complexity and cost. Some SAF pathways may be more suitable than others in certain areas of the world depending on feedstock availability and processing capabilities.

A number of additional pathways are going through the vigorous assessment process and are expected to be approved in the coming years.

Pathway Feedstock Certification name Blend limit
Fischer-Tropsch Energy crops, lignocellulosic biomass, solid waste FT-SPK Up to 50%
Hydroprocessed Esters and Fatty Acids (HEFA) Waste fats, oils, greases (FOGs) from vegetable and animal sources HEFA-SPK Up to 50%
Direct Sugars to Hydrocarbons (DSHC) Conventional sugars, lignocellulosic sugars HFS-SIP Up to 10%
Fischer-Tropsch with Aromatics Energy crops, lignocellulosic biomass,
solid waste
FT-SPK+A Up to 50%
Alcohol-to-Jet (AtJ) Sugar, starch crops, lignocellulosic biomass ATJ-SPK Up to 50%
Catalytic Hydrothermolysis Jet (CHJ) Waste fats, oils, greases (FOGs)
from vegetable and animal sources
CHJ or CH-SK Up to 50%
HEFA from Algae Micro-algae oils HC-HEFA-SPK Up to 10%
FOG Co-Processing Waste fats, oils, greases (FOGs)
from vegetable and animal sources
FOG-CP Up to 5%
FT Co-Processing Fischer-Tropsch biocrude FT-CP Up to 5%

For more information about the various production pathways for SAF, download the Beginner's Guide to Sustainable Aviation Fuel.

Making sure SAF is fit to fly

SAF must have the same chemical qualities and characteristics as conventional jet fuel. This is important to ensure that manufacturers do not have to redesign engines or aircraft, and that fuel suppliers and airports do not have to build new fuel delivery systems. To ensure technical and safety compliance, SAF must undergo strict laboratory, ground and flight tests under an internationally-recognised standard. Once the SAF has demonstrated compliance with the requirements, it is blended with conventional jet fuel and re-tested.

Why does a blend limit exist?

At present, SAF must be blended with conventional jet fuel (up to 50%). Some less environmentally favourable components of conventional jet fuel (e.g. sulphur) allow seals to swell in engines and prevent fuel leaks. Newer engines do not have this concern, and SAF has been performance tested at 100% in military aircraft. It is likely that higher blend limits will be approved in the future as synthetic aromatic compounds are approved for use and as major aircraft manufacturers work to ensure aircraft are compatible with 100% SAF by around 2030.


Safety is the aviation industry’s top priority, therefore the process for testing potential new fuels is extremely rigorous. Through testing in laboratories, in equipment on the ground, and under the extreme conditions of in-flight operations, an exhaustive process determines the suitability of SAF.

Today's SAF has similar properties to conventional jet fuel, Jet A-1. Tests look at specific fuel consumption at several power settings, from ground idle to take-off speed, which is then compared to performance with conventional jet fuel. The amount of time it takes for the engine to start, how well the fuel stays ignited in the engine, and how the fuel performs in acceleration and deceleration, are all tested thoroughly. Tests are also completed to ensure fuels don’t have a negative impact on the materials used in building aircraft and components.

Once the lab and ground tests have been completed, the fuel is tested on aircraft under normal operating conditions. During the test flight, pilots perform a number of standard tests, as well as simulating exceptional circumstances, to ensure the fuel can withstand use under any operating conditions.

The aerospace sector is currently working and testing 100% SAF, to ensure aircraft can be operated on it without the current blend limits.